The ability to isolate and manipulate plant genes has opened the way to gain understanding about the mechanisms involved in the regulation of plant gene expression. This knowledge is important for the exploitation of genetic engineering techniques, applied to problems such as the expression of genes in genetically manipulated crop plants. A large number of examples are now in the literature of plant DNA sequences which have been used to drive the expression of foreign genes in plants. In most instances the regions immediately 5' to the coding regions of genes have been used in gene constructs. These regions are referred to as promoter sequences. They may be derived from plant DNA; or from other sources, e.g., viruses. It has been demonstrated that sequences up to 500-800 bases in most instances are sufficient to allow for the regulated expression of foreign genes. This regulation has involved tissue-specificity; regulation by external factors such as light, heat treatment, chemicals and hormones; and developmental regulation.
These experiments have been carried out using gene fusions between the promoter sequences and foreign genes such as bacterial promoter genes, etc.
Although regulation has been observed this has been hampered by two factors:
1. The low level of expression observed for the transgene in comparison with the endogenous gene. In most instances expression of the transgene has been approximately 1-10% of the expression achieved when the same promoter drives the endogenous gene. This has led to the suggestion that sequences internal to genes may also be important for efficient expression. This has been supported by experiments in which complete genes including 5' and 3' regions as well as coding regions have been used in blot transformation experiments. The influence of sequences surrounding the introduced transgene on the level of expression which can be achieved is normally referred to as `position effect`. For practical purposes it is desirable that gene constructs introduced into plants give expression levels comparable with that of an endogenous gene. In practice, promoters may be chosen for gene constructs because of their induction pattern, e.g. their tissue specificity or temporal pattern of expression. However, the level of expression of the transgene is usually critical; if the desired promoter cannot give a high enough level of expression it will not be useful.
2. Great variation exists in the level of expression of transgenes between different transformed plant lines. It is not clear why this should be so: it may be another manifestation of the "position effect". These expression levels can differ by as much as two orders of magnitude. Thus a large number of transformants may need to be analysed before one exhibiting the desired expression level can be identified.
These two factors make it very costly and time-consuming to use known promoter constructs for practical genetic plant engineering.